The present invention relates to a thermal transfer sheet, particularly to a thermal transfer sheet of a novel co-winding type wherein a thermal transfer sheet and a transfer receiving material have been temporarily bonded to each other.
Hitherto, in a case where output from a computer or a word processor is printed by use of a thermal transfer system, there has been used a thermal transfer sheet comprising a substrate film and a heat-fusible ink layer disposed on one surface side thereof.
Such a conventional thermal transfer sheet comprises a substrate film comprising a paper having a thickness of 10 to 20 .mu.m such as a capacitor paper and a paraffin paper, or comprising a plastic film having a thickness of 3 to 20 .mu.m such as a polyester film and a cellophane film. The above-mentioned thermal transfer sheet has been prepared by coating the substrate film with a heat-fusible ink comprising a wax and a colorant such as a dye or a pigment mixed therein, to form a heat-fusible ink layer on the substrate film.
When printing is effected on a transfer receiving material by using such a conventional thermal transfer sheet, the thermal transfer sheet is supplied from a roll thereof, while a continuous or sheet-like transfer-receiving material is also supplied, so that the former and the latter are superposed on each other on a platen. Then, in such a state, heat is supplied to the thermal transfer sheet from the back side surface thereof by means of a thermal-head to melt the ink layer and transfer it to the transfer receiving material, whereby a desired image is formed.
However, even when the above-mentioned conventional thermal transfer sheet is as such intended to be used in a facsimile printer using a conventional thermal (or heat-sensitive) color-developing (or color-forming) paper, the thermal transfer sheet cannot be used in such a large size plotter since the above plotter does not include a conveying device for a transfer-receiving material.
In order to solve the above-mentioned problem, there has been proposed a method wherein a thermal transfer sheet and a transfer-receiving material are temporarily bonded to each other in advance and wound into a roll form so that the thermal transfer sheet may be adapted to a plotter, etc., or the device to be used in combination therewith may be simplified or miniaturized.
However, when an image is formed by using a tracing paper as the transfer-receiving material for the above co-winding type thermal transfer sheet and the tracing paper carrying thereon the thus formed image is used as an original image so as to provide a blueprint image, the line image portion constituting the resultant image is blurred. As a result, there has been posed a problem such that a blue-print image having a high precision cannot be formed. Particularly, when a transparent drum containing therein a light source for a copying machine is heated up to a high temperature on the basis of the accumulation of heat, a portion of the ink constituting the original image is transferred to the drum, thereby to pose a problem such that a large number of spots or dots are produced in the copied image formed after such a transfer of the ink. In addition, there has also been posed a problem such that the thus formed blue-print image only provides a small contrast.
On the other hand, an overhead projection (hereinbelow, sometimes referred to as "OHP") has widely been used in various meetings such as lecture meeting, class or school meeting and explanatory meeting. A transparent sheet (hereinafter, referred to as "OHP sheet") to be used for the OHP comprises a sheet or film having a thickness of several tens of microns to several hundreds of microns and predominantly comprising a transparent resin such as polyester and polypropylene. In order to form an image on such an OHP film or sheet, there has been used a method such as hand writing, printing and thermal (or heat-sensitive) transfer method.
When an image is intended to be formed on the above OHP sheet by use of a thermal transfer method, it is possible to separately feed a thermal transfer sheet and an OHP-sheet to a printer. However, since the OHP sheet is generally of a sheet type, it is preferred to use a so-called "co-winding type thermal transfer sheet" comprising an OHP sheet and a thermal transfer sheet which has temporarily been bonded to the surface of the OHP sheet in advance so that these sheets are peelable from each other. When such a co-winding type thermal transfer sheet is used, it is possible to form an OHP image (or image to be used for the OHP) by use of a simple printer.
However, in general, the OHP sheet is considerably hydrophobic and therefore it is difficult to well bond the OHP sheet and the thermal transfer sheet to each other so that they are peelable.
Further, when the thus prepared co-winding type thermal transfer sheet is stored for a certain period of time and thereafter an image is formed on an OHP sheet by use of the thus stored co-winding type thermal transfer sheet, the OHP sheet is contaminated with small fragments of the ink layer of the thermal transfer sheet and the pigment dropped out of the ink layer, so that the entirety of the OHP sheet becomes dark or blackish.
Furthermore, in general, the resultant image formed from the above co-winding type thermal transfer sheet or the resultant OHP sheet carrying thereon such an image has a smooth surface and is lacking in a liquid absorbing property and therefore the heat-fusible ink does not sufficiently penetrate or permeate the OHP sheet, so that the thus formed ink image is liable to be easily peeled from the OHP sheet, i.e., the resultant wear resistance of the ink image is liable to pose a problem. Such a problem has been encountered not only in the OHP sheet or tracing paper but also in most of opaque or colored plastic sheets or films, metal foils, etc.
On the other hand, when an image having at least two colors is intended to be formed by use of a thermal transfer sheet, it is preferred that the thermal transfer sheet and a heat-sensitive color developing paper are temporarily bonded to each other in advance, and the resultant laminate is rolled into a roll form (i.e., a co-winding roll). In the case of such a co-winding type thermal transfer sheet, it is required to have various performances such that the thermal transfer sheet is tightly bonded to the thermal color developing paper so as to provide no wrinkle or deviation, both of these are easily peeled from each other after thermal transfer operation, the ink layer is exactly transferred to the thermal color developing paper in the transfer region, and the ink layer is not transferred to the thermal color developing paper at all in the non-transfer region so that the paper is not contaminated. However, the conventional co-winding type thermal transfer sheet does not fully satisfy such requirements.
In addition, various curtains, outdoor displays, flags, etc., wherein large characters have been written on a cloth or fabric, etc., by use of India ink and a brush, are widely used for the purpose of advertising, publicating or propaganda, or various events or functions such as ceremonial occasions (i.e., coming-of-age ceremonies, weddings, funerals, festivals, etc.) In a case where characters are written on the cloth or fabric by use of the India ink and a brush in the manner as described above, when the same characters are written on a large number of cloths or fabrics, a printing process may be used. However, when some characters are written for the purpose of a funeral which cannot be expected in advance, and different characters are written on different cloths or fabrics, considerable trouble is required. Further, at present, it is difficult to find a person who is capable of well writing (i.e., is good at handwriting), and therefore many problems are liable to occur.